1. Field of the Invention
The invention in general relates to the testing of sonar systems, and particularly to an arrangement of the type which eliminates need for movement of the sonar system through the water environment.
2. Background Information
Vehicle-carried sonar systems which travel through the water environment are limited in performance by virtue of unwanted noise which is generated during the course of travel. A predominant source of noise is flow noise caused by turbulence in the boundary layer surrounding the moving sonar array and as speed of movement through water increases, the flow noise may become the dominant noise in a well designed sonar receiver, particularly when operation is at the lower frequencies.
The detrimental effects of flow noise can be significantly reduced by a number of techniques such as streamlining of the carrying vehicle, proper placement of the transducers of the sonar array on the vehicle and geometric or amplitude shading of the transducers, to name a few. If the response of the sonar system to the flow noise can be determined, corrective measures may be taken. The performance of the sonar system can be significantly increased by reducing the effects of flow induced noise.
Measuring flow noise and flow noise performance of a sonar system in the field is a technically difficult and very expensive undertaking. For example, propelling the sonar array through the water on a carrying vehicle creates not only flow noise but additionally, propulsion noise which is difficult, if not impossible, to separate from the flow noise. To eliminate propulsion noise complications, the array may be given what is known as a pop-up test wherein the sonar array is mounted on a buoyant vehicle that is pulled to the bottom of a body of water and released to rise to the surface using its own buoyancy as the propulsive force. Although this eliminates the propulsion noise such tests are very expensive and require a variety of specially designed and instrumented pop-up vehicles.
In order to eliminate the requirement for in situ testing, an alternative approach in common use is that of static test equipment. With such arrangement, a calibration array of test transducers is assembled, with the transducers of the calibration array being placed over the transducers of the sonar array under test, with an acoustic coupling medium between them. The movement of turbulence over the array under test is simulated by providing the test transducers with test signals indicative of a dynamic representation of moving vortices, such test signals being known a priori. Although this test arrangement provides some advantages, the calibration array to match the sonar array under test is very expensive and in many instances may equal or exceed the expense of the array under test. Further, a calibration array made for one system will not generally be applicable to other arrays of different size and shape.
It is an object of the present invention to provide a sonar test system which is more or less universal in its ability to test different sonar arrays, and at reduced expenses.